a. Field of the Invention
This invention relates to a fixing device which uses a pair of fixing rollers for heating and fixing a thermally fusible, visualized image on a copy medium.
B. Description of the Prior Art
The conventional fixing device using a pair of fixing rollers of the described type has a heating roller for fixing a toner image on a copy medium and a press roller for urging the copy medium against the heating roller, and as a construction thereof, there is known a two-roller construction comprising a heating roller and a press roller or a three-roller construction comprising a heating roller, a press roller and a heating roller.
Usually, these heating and press rollers comprise a rigid and an elastic roller, respectively, and are urged into contact with each other so that the time for fixation of the copy medium may be afforded by an increase in the area of contact between the rollers resulting from the deformation of the elastic roller.
Various methods of imparting a pressure to the elastic material are known (for example, those using a cam, a link or air pressure), but no particular attention has been paid to the method of releasing the pressure. Even when the power supply to the device happens to be cut off for example, when the electric current to the device it cut off due to service interruption or other accident during copying operation, the elastic roller would suffer from a permanent distortion because of the pressure and residual heat unless it is released from its pressed state, thus resulting in seriously hampered durability of the device.
Therefore, the pressure release is indispensable when the device has been stopped and this has given rise to the need for a fixing device which has covered such a problem.